Current wire bonders typically have bondheads comprising rocker arms that rotate a bonding tool up and down about a horizontal axis with a small angular stroke. The angular stroke of the rocker arm positions the bonding tool along a vertical Z-axis. The bondhead is mounted on mutually orthogonal X and Y motion stages of an XY table in order to position the bonding tool in the X and Y axes on a horizontal plane.
The said Z-axis motion of the bonding tool is usually driven by a direct drive motor, such as a voice coil motor. For instance, U.S. Pat. No. 7,025,243 entitled “Bondhead for Wire Bonding Apparatus” discloses a bonding tool held by a bondhead body, which is driven by a bondhead actuator to rotate in order to position the bonding tool with respect to a bonding surface.
FIG. 1 is a side view of a prior art wire bonder bondhead that is mounted on an XY table. The wire bonder generally comprises a bondhead body 10 to which a bonding tool such as an ultrasonic transducer 12 is mounted for generating ultrasonic bonding energy. The bondhead body 10 is enclosed in a bondhead housing 14 and it is rotatably positioned on a pivot, such as a flexure bearing 16, to create the aforesaid rocker arm mechanism. The bondhead body 10 and ultrasonic transducer 12 are drivable to rotate about the X-axis by a direct drive actuator such as a voice coil motor 18. A wire clamp 20 is located over the ultrasonic transducer 12 for feeding bonding wire 22 to a capillary 24 at a tip of the ultrasonic transducer 12. The capillary 24 attaches the bonding wire 22 to bonding surfaces of a die 26, and a carrier 28 on which the die 26 is mounted.
The base of the bondhead housing 14 is mounted on an XY table 30, which comprises an X-stage to linearly drive the bondhead body along an X-axis and a Y-stage to linearly drive the bondhead body 10 along a Y-axis. The X and Y stages of the XY table are separately driven by direct drive motors called linear motors. The various stages for driving X, Y and Z motion of the bonding tool are preferably equipped with suitably mounted encoders which provide very high resolution position feedback for closed loop control of the bonding tool to ensure bonding accuracy at the tip of the capillary 24.
The speed of wire bonding machines has increased year by year. This has resulted in higher force (and power) requirements from the direct drive motors. Since one stage (typically the X-stage) carries the other stage (typically the Y-stage) which in turn carries the bondhead including the Z-stage, it is the X-stage which has to move the largest mass at high accelerations. This makes the X-motor the bulkiest component and also results in large amounts of heat being dissipated by the X-motor during wire bonding operations. The resulting high temperature tends to reduce the reliability of the X-motor. Also, the heat generated needs elaborate cooling arrangements to prevent it from migrating to the bonding area, which may affect the accuracy of bonding operations.
Due to higher speeds of the XY table, which in turn require higher accelerations, the vibrations generated by motion of the moving mass have also increased considerably. These vibrations are transmitted to the work-holder which holds the substrate or carrier being bonded, thus adversely affecting bond-placement accuracy on the same. Since the X-stage has to move the largest mass, the vibrations created by the X-stage have the highest magnitude.
Another observation is that the linear bearings of the X-stage are the worst stressed, due to high preloading for high stiffness and also due to the high moment loading resulting from the offset between an actuating X-force and the shifting centre of gravity of the mass carried on it that is moving in the Y-direction.
With a view to overcoming some of the above problems, U.S. Pat. No. 6,460,751 entitled “Bondhead for a Wire Bonder” describes a wire bonding apparatus in which the linear X-stage has been altogether eliminated. Instead a rocker-arm rotary stage is mounted on a vertical axis rotary stage. The rotary stage uses air bearings and is driven by a direct drive motor. The rotary stage, with a vertical rotary axis, is mounted onto the linear Y-stage. Such a rotary stage in effect replaces the X-stage but does not impart purely linear motion in the X-direction to the bonding tool. Since the motion of the bonding tool is rotary, it has an X-direction component as well as a Y-direction component. The Y-direction component can be compensated for by the linear Y-motion imparted by the Y-stage.
Although this design is meant to solve the aforementioned problems associated with the conventional X-stage, it has its own limitations. Firstly, since the total angular travel range is relatively large (+/−15 degrees), the force that is required of the direct drive motor—although less than the purely linear X-stage that it replaces—is still quite substantial with the result that the direct drive motor cannot be made very small and compact. Furthermore, use of air bearings places very high demands on the precision of the manufactured parts and their assembly. It also takes up quite considerable space. Being relatively heavy, it increases the loading on the Y-stage which may then begin to face problems similar to the ones mentioned above for the X-stage. Air-bearings also consume copious amount of compressed air even when the bonder is not bonding, thus adding to the running costs.